1. Field of the Invention
The present invention relates to a power unit of an automobile, and more particularly to an automobile power unit including an engine provided with a mechanical supercharger of a type which is driven by the engine output power, and a power transmission having a hydrodynamic power transmitting device, such as a torque converter or a fluid coupling, which is provided with a lock-up mechanism. More specifically, the present invention pertains to a control of the aforementioned type of power unit.
2. Description of Prior Art
Proposals have already been made of an engine provided with a mechanical supercharger which is driven by the engine output under a specific engine operating condition, such as a heavy load, high speed engine operation, to provide a high engine output power. For example, Japanese utility application No. 55-188933 filed on Dec. 27, 1980, and disclosed for public inspection on July 10, 1982, under the disclosure No. 57-112033, discloses an engine having an intake passage provided with a supercharger which is driven by the engine output shaft through an electromagnetic clutch so that the supercharger is driven under a heavy load range. However, as pointed out in the Japanese utility model application, this type of supercharger control produces a stepwise change in the engine output torque between the supercharged zone and the non-supercharged zone. The utility model proposes to solve the problem by relieving a controlled amount of the supercharger outlet air to the supercharger inlet so that a smooth engine output change can be established. The solution is not, however, recommendable because the partial relief of the supercharged air may cause an energy loss.
The inventor has found that, in the case of an automobile having an automatic transmission provided with a hydrodynamic power transmitting device, such as a torque converter or a fluid coupling, the aforementioned stepwise change in the engine output torque does not cause any serious problem because the hydrodynamic device can absorb the stepwise engine output torque change. It should however be noted that, in a recent development in the field of automatic transmissions, the hydrodynamic device is provided with a lock-up clutch for directly connecting the input and output members. As taught by U.S. Pat. No. 4,027,554 issued on June 7, 1977 to Ito et al, the lock-up clutch may be engaged when the rotating speed of the output member of the hydrodynamic device is increased beyond a predetermined value. In case where the hydrodynamic device is provided with a lock-up clutch and the lock-up clutch is engaged, the aforementioned stepwise change in the engine output torque may cause a shock to be transmitted to the automobile body.